If you get this error message:

mount: unknown filesystem type 'vboxsf'

First be sure that you have installed the last version of Virtual Box and the module "Guest additions".

Ensure that your user name is on vboxsf on the host

$sudo gpasswd -a host_user_name vboxsf

You need to be authorized to mount the synced files, once your username is on the group the problem is solved.

Another solution is the one given by VirtualBox

sudo ln -s /opt/VBoxGuestAdditions-4.3.10/lib/VBoxGuestAdditions /usr/lib/VBoxGuestAdditions

Source from Mint Forum

Closed issue from official site

Source from VirtualBox